2025ApJ...978..145N

The 21 cm forest is a sensitive probe for the early heating process and small-scale structures during the epoch of reionization (EOR), to be realized with the upcoming Square Kilometre Array (SKA). Its detection relies on the availability of radio-bright background sources, among which the radio-loud quasars are very promising, but their abundance during the EOR is still poorly constrained due to limited observations. In this work, we use a physics-driven model to forecast future radio-loud quasar observations. We fit the parameters of the model using observational data of high-redshift quasars. Assuming Eddington accretion, the model yields an average lifetime of t_q ∼ 10^5.5 yr for quasars at z ∼ 6, consistent with recent results obtained from quasar proximity zone observation. We show that if the radio-loud fraction of quasars evolves with redshift, it will significantly reduce the abundance of observable radio-loud quasars in the SKA era, making 21 cm forest studies challenging. With a constant radio-loud fraction, our model suggests that a 1 yr sky survey conducted with SKA-LOW has the capability to detect approximately 20 radio-loud quasars at z ∼ 9, with sufficient sensitivity to resolve individual 21 cm forest lines.